Step down gear trains are more specifically utilized for automotive vehicle seats provided with motor-driven adjusting device, for example for the height adjusting device of the seat, the inclination adjusting device of the seat back, the lengthwise or inclination adjusting device of the seat's front edge. Such type adjusting devices require a relatively slow rotating drive, electric motors however rotate at a relatively high speed. Accordingly, the step down gear train must have a sufficiently high gear reduction ratio. In principle, the step down gear train of the type mentioned herein above is also suited for hand driven adjusting devices.
The pinion on the output shaft of the step down gear train usually interengages with a toothed surface area such as a sector gear or a toothed rack. In order for the adjusting device to be clearance free, it must be made certain that, when the drive is idle, the pinion has a fixed position, that is with zero clearance in the one or in the other direction of rotation. Adjusting devices of automotive vehicle seats are also loaded in the load return path. It is to be made certain that the pinion is clearance free not only when it is not loaded but in the loaded condition as well. It is thus made certain that the adjusting device itself is also locked in a defined way and can be maintained in the adjusted position with zero clearance.
Typically, step down gear trains are more loaded in one direction of rotation in the load return path than in the other direction of rotation. A seat height adjusting device is for example loaded by the weight of an occupant in the load return path in the direction which corresponds to lowering the seat, but is normally not loaded in the opposite direction. Crash loads however often act in the opposite direction, and in most cases only in one direction of rotation.
Known step down gear trains usually have a considerable clearance at the pinion. This clearance often is too great for such a step down gear train being utilized in adjusting devices of automotive vehicles and cannot be tolerated. In the case of inclination adjusting devices for the seat back of an automotive vehicle seat, the seat back constitutes a quite long lever arm that is pivotal about the axis of the adjusting device. In the upper region of the seat back no clearance is allowed to be noticed, meaning the seat back is not allowed to be loose.